Steered fiber orientation: correlating orientation and residual tensile strength parameters of SFRC

Abstract

Adding steel fibers to concrete improves the post-cracking tensile strength of the composite material due to fibers bridging the cracks. The residual performance of the material is influenced by fiber type, content and orientation with respect to the crack plane. The latter is a main issue in fiber-reinforced concrete elements, since it significantly influences the structural behavior. The aim of this research is to develop a tailor-made composite material and casting method to orient fibers in order to optimize the performance of the material for structural applications. To this aim, a mechanized concreting device that induces such preferred fiber orientation is designed and fabricated. It uses vibration and a series of narrow channels to guide and orient fibers. A composite with oriented fibers is produced using a hybrid system of macro and micro fibers and high-performance concrete. From the same concrete batch, specimens are cast both with and without the fiber orientation device, obtaining different levels of fiber orientation. Three-point bending tests are performed to measure and compare the residual tensile strength capacities with standard specimens cast according to EN 14651. Elements with favorable fiber orientation show a significant increase in residual tensile strength with respect to the standard beams. Finally, computed tomography and an electromagnetic induction method are employed to better assess the orientation and distribution of fibers in the beams. Their results are in good agreement and enable to link the residual tensile strength parameters with fiber orientation.

Influence of fiber orientation on flexural‐based design of steel fiber‐reinforced concrete slabs: Experimental and numerical program

“…The orientation factor (K) can be introduced in the design procedure in order to take into account the difference between the fiber orientation (η θ ) in a real structure and small-scale beams used for the material characterization. [26][27][28][29][30] In this regard, the orientation numbers that correspond to each of both elements have a major influence on the magnitude of K. Recent studies and recommendations provide empirical, 58,60,61 analytical, 60,62 and numerical approaches 52,63,64 to carry out the assessment of K once the magnitudes η θ of are quantified.…”

“…One of the main reasons behind this phenomenon is a more favorable fiber orientation along the main (longitudinal) direction of the small‐scale beams in comparison with large‐scale structures 26–30 . In this regard, the fib Model Code 2010 22 and Annex L to the new Eurocode 2 31 propose an orientation factor ( K , in accordance with Clause 5.6.7 of the fib Model Code 2010 22 ) that reduces/increases the serviceability and ultimate residual tensile strength of the FRC element once unfavorable/favorable fiber orientation is experimentally verified.…”

“…1,7,25,26 One of the main reasons behind this phenomenon is a more favorable fiber orientation along the main (longitudinal) direction of the small-scale beams in comparison with large-scale structures. [26][27][28][29][30] In this regard, the fib Model Code 2010 22 and Annex L to the new Eurocode 2 31 propose an orientation factor (K, in accordance with Clause 5.6.7 of the fib Model Code 2010 22 ) that reduces/ increases the serviceability and ultimate residual tensile strength of the FRC element once unfavorable/favorable fiber orientation is experimentally verified. Both documents, which are references for the design of SFRC structures, declare-within the respective background documents-K to be dependent on numerous variables, such as: casting and compaction processes, fresh-state properties of the concrete mix, fiber, and formwork type/ geometry.…”

Hybrid assessment of fiber orientation influence on the load resistance of anchors in SFRC

“…Some possibilities can be found with dependence on controlling the concreting process, for example, by pouring direction, 23 mix viscosity, 21 formwork configuration, 24 spraying, 25 and separate fiber dosing. 26 Specialized techniques and technologies include the attachment of steel fibers to flexible warps within the formwork, 27 application of magnetic fields 28 at the formwork circumference, manual 29 and mechanical layering of concrete, 30 but also robot-assisted localization of the fibers at pouring, 31 or in additive manufacturing. 32 The potential of fiber orientation as an influence on the loadbearing behavior of anchorages in concrete has been recognized and conceptually described in the study by Holschemacher et al 33,34 Respectively, to account for the beneficial effects of fiber reinforcement on the anchor resistance, the design recommendation by Vita et al 16,18 requires that the anchor depth is at least more than 1.7 times the fiber length, that is, it reaches below a zone of nonuniform fiber orientation.…”

“…Some possibilities can be found with dependence on controlling the concreting process, for example, by pouring direction, 23 mix viscosity, 21 formwork configuration, 24 spraying, 25 and separate fiber dosing 26 . Specialized techniques and technologies include the attachment of steel fibers to flexible warps within the formwork, 27 application of magnetic fields 28 at the formwork circumference, manual 29 and mechanical layering of concrete, 30 but also robot‐assisted localization of the fibers at pouring, 31 or in additive manufacturing 32 …”